Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly ("MEA") which comprises an ion exchange membrane or solid polymer electrolyte disposed between two electrodes formed of porous, electrically conductive sheet material, typically carbon fiber paper. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes to an external load.
At the anode, the fuel permeates the porous electrode material and reacts at the catalyst layer to form cations, which migrate through the membrane to the cathode. At the cathode, the oxygen-containing gas supply reacts at the catalyst layer to form anions. The anions formed at the cathode react with the cations to form a reaction product.
In electrochemical fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of hydrogen ions from the anode to the cathode. In addition to conducting hydrogen ions, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode, oxygen reacts at the catalyst layer to form anions. The anions formed at the cathode react with the hydrogen ions that have crossed the membrane to form liquid water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:
Anode reaction: H.sub.2 .fwdarw.2H.sup.+ +2e.sup.- PA1 Cathode reaction: 1/2O.sub.2 +2H.sup.+ +2e.sup.31 .fwdarw.H.sub.2 O PA1 (a) a fuel cell stack comprising a fuel stream inlet, a fuel stream outlet, an oxidant stream inlet, and at least one fuel cell for promoting an electrocatalytic reaction of a fuel stream introduced at the fuel stream inlet with an oxidant stream introduced at the oxidant stream inlet to produce electricity, reaction product, and heat; PA1 (b) a pressurized fuel supply having a pressure control valve for regulating the pressure of the fuel supply; PA1 (c) a vacuum ejector interposed between the fuel supply and the fuel stream inlet, the ejector comprising a motive inlet, a suction inlet, and a discharge outlet, the motive inlet fluidly connected to the fuel supply, the suction inlet fluidly connected to the fuel stream outlet, and the discharge outlet fluidly connected to the fuel stream inlet; PA1 (d) a pressure transducer interposed in the fuel stream between the discharge outlet and the suction inlet, the pressure transducer detecting the pressure of the fuel stream and transmitting a corresponding signal to the pressure control valve; and PA1 (e) a pressurized oxidant supply fluidly connected to the oxidant stream inlet. PA1 (a) a fuel cell stack comprising a fuel stream inlet, an oxidant stream inlet, an oxidant stream outlet, and at least one fuel cell for promoting an electrocatalytic reaction of a fuel stream introduced at the fuel stream inlet with an oxidant stream introduced at the oxidant stream inlet to produce electricity, reaction product, and heat; PA1 (b) a pressurized oxidant supply having a pressure control valve for regulating the pressure of the oxidant supply; PA1 (c) a vacuum ejector interposed between the oxidant supply and the oxidant stream inlet, the ejector comprising a motive inlet, a suction inlet, and a discharge outlet, the motive inlet fluidly connected to the oxidant supply, the suction inlet fluidly connected to the oxidant stream outlet, and the discharge outlet fluidly connected to the oxidant stream inlet; PA1 (d) a pressure transducer interposed in the oxidant stream between the discharge outlet and the suction inlet, the pressure transducer detecting the pressure of the oxidant stream and transmitting a corresponding signal to the pressure control valve; and PA1 (e) a pressurized fuel supply fluidly connected to the fuel stream inlet. PA1 (a) interposing a vacuum ejector between the first reactant supply and the first reactant stream inlet of the stack, the ejector comprising a motive inlet, a suction inlet, and a discharge outlet; PA1 (b) fluidly connecting the motive inlet to the first reactant supply; PA1 (c) fluidly connecting the suction inlet to the first reactant stream outlet of the stack; PA1 (d) fluidly connecting the discharge outlet to the first reactant stream inlet; PA1 (e) interposing a pressure transducer between the discharge outlet and the first reactant stream inlet, the pressure transducer capable of detecting the pressure of the first reactant stream and transmitting a corresponding signal to the pressure control valve; PA1 (f) transmitting a signal from the pressure transducer to the pressure control valve to increase the pressure of the first reactant supply when the detected pressure of the first reactant stream falls below a predetermined value; and PA1 (g) transmitting a signal from the pressure transducer to the pressure control valve to decrease the pressure of the first reactant supply when the detected pressure of the first reactant stream exceeds a predetermined value. PA1 (h) interposing a pressure transducer between the second reactant supply and the second reactant stream inlet of the stack, the second reactant pressure transducer capable of detecting the pressure of the second reactant stream and transmitting a corresponding signal to the pressure control valve; PA1 (i) transmitting a signal from the pressure transducer to the pressure control valve to increase the pressure of the first reactant supply when the detected pressure of the second reactant stream increases; and PA1 (j) transmitting a signal to the pressure control valve to decrease the pressure of the first reactant supply when the detected pressure of the second reactant stream decreases.
In typical fuel cells, the MEA is disposed between two electrically conductive plates, each of which has at least one flow passage engraved or milled therein. These fluid flow field plates are typically formed of graphite. The flow passages direct the fuel and oxidant to the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. In a single cell arrangement, fluid flow field plates are provided on each of the anode and cathode sides. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of water formed during operation of the cell.
Two or more fuel cells can be connected together, generally in series but sometimes in parallel, to increase the overall power output of the assembly. In series arrangements, one side of a given fluid flow field plate serves as an anode plate for one cell and the other side of the fluid flow field plate can serve as the cathode plate for the adjacent cell. Such a series connected multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together in its assembled state by tie rods and end plates. The stack typically includes manifolds and inlet ports for directing the fluid fuel stream (substantially pure hydrogen, methanol reformate or natural gas reformate) and the fluid oxidant stream (substantially pure oxygen or oxygen-containing air) to the anode and cathode flow field channels. The stack also usually includes a manifold and inlet port for directing the coolant fluid stream, typically water, to interior channels within the stack to absorb heat generated by the exothermic reaction of hydrogen and oxygen within the fuel cells. The stack also generally includes exhaust manifolds and outlet ports for expelling the unreacted fuel and oxidant gases, each carrying entrained water, as well as an exhaust manifold and outlet port for the coolant water exiting the stack.
Solid polymer fuel cells generally employ perfluorosulfonic ion exchange membranes, such as those sold by DuPont under its NAFION trade designation and by Dow under the trade designation XUS 13204.10. When employing such membranes, the fuel and oxidant reactant streams are each generally humidified before they are introduced to solid polymer fuel cells so as to facilitate cation exchange and to avoid drying, and thus damaging, the ion exchange membranes separating the anode and cathode of each cell.
Each of the fuel cells making up the stack is typically flooded with the selected fuel and oxidant at a desired pressure. The pressure is generally controlled by a regulator at the source of the reactant. When an electrical load is placed on the circuit connecting the electrodes, the fuel and oxidant are consumed in direct proportion to the electrical current drawn by the load.
Each reactant stream exiting the fuel cell stack generally contains water. The outlet fuel stream from the anodes generally contains the water added to humidify the stream plus any product water drawn across the membrane from the cathode and absorbed as vapor in the fuel stream. The outlet oxidant stream from the cathodes generally contains the water added to humidify the stream plus product water formed at the cathode that is either entrained as water droplets or is absorbed as vapor in the oxidant stream. As the power output of the fuel cell stack is increased, more water accumulates at the anode and at the cathode, thereby increasing the recirculation flow rate required to remove water and keep the flow channels in the stack unobstructed.
Excess water extracted from one or both of the reactant streams exiting the fuel cell can be accumulated in a separator or knockout drum. The excess water so accumulated can then be recirculated and used as a source of coolant fluid or humidification water, or simply drained from the system.
When one of the reactants fed to the fuel cells is substantially pure hydrogen or oxygen, the unconsumed reactant exhausted from the fuel cells may be recirculated to minimize waste that would result from venting the reactant to the atmosphere. Excess water may be removed from the recirculated reactant stream before it is merged with the corresponding incoming fresh reactant stream upstream of the inlet to the fuel cell stack. Alternatively, the recirculated reactant stream containing water vapor may be merged directly with the incoming fresh reactant stream, thereby humidifying the incoming fresh reactant stream and avoiding the need for a separate humidifier.
Similarly, when one or both of the reactants is a dilute reactant, such as a reformate or air, the unconsumed reactant exhausted from the fuel cells may also be recirculated, particularly in the case of the fuel stream. However, the dilute reactant stream is more often discarded after it has passed once through the fuel cell stack, particularly when the dilute reactant is air. The excess water in the outlet dilute reactant stream is generally removed in a separator or knockout drum. The exhaust reactant stream is then generally vented to the atmosphere.
It is often advantageous to integrate the product water separated from the outlet reactant streams with the coolant stream, and thereby use the product water generated electrochemically in the fuel cell stack to regulate the temperature of the stack. In this regard, the use of product water as the coolant avoids the need to provide a separate external source of coolant fluid, since the water generated by the fuel cells is itself a suitable coolant fluid.
In characterizing systems employing recirculated reactant streams, it is convenient to define the term "recirculation ratio". As used herein, "recirculation ratio" is the amount of a reactant supplied to the fuel cell stack divided by the amount of the reactant consumed in one pass through the fuel cell stack. In typical hydrogen/oxygen fuel cell stacks, the hydrogen recirculation ratio ranges from 1.2 to 5.0, and more preferably from 1.5 to 2.0.
In fuel cell based electric power generation systems in which one or more of the reactant streams is recirculated, vacuum ejectors have been employed to effect recirculation. Winters U.S. Pat. No. 3,462,308 discloses a fuel cell system in which each of the fuel and oxidant streams discharged from the fuel cell is recirculated and merged with the respective incoming, fresh fuel and oxidant streams by means of ejectors 23 and 23'. Each ejector is described as including a venturi throat. However, Winter's ejector configuration is designed for fixed-point operation in that a constant reactant stream pressure drop is required across each of the ejectors. In order to maintain the necessary pressure drop across the ejectors to effect recirculation, Winter's system discharges the recirculated reactant streams as required via vent valves 21 and 21'. Thus, Winter's reactant recirculation system has a load-following capability, but incurs a serious efficiency penalty due to the venting of the recirculated reactant streams to the atmosphere.
Vacuum ejectors have also been incorporated into the fuel processing subsystem of a reformate-based fuel cell electric power generation system. In Fanciullo et al. U.S. Pat. No. 3,745,047, an ejector is employed to draw steam into a fuel stream prior to its introduction into a reformer. In the Fanciullo system, however, an ejector is not employed to recirculate the fuel stream (or the oxidant stream), since the outlet fuel stream is not recirculated to the fuel cell but is instead directed through conduit 34 to the reformer burner.
The primary purpose of an ejector is to transport a gas, liquid, powder or solid particles from one pressure level to a relatively higher pressure level. Ejectors generally contain no moving parts and are therefore considered to be passive devices. In an ejector, pressurized motive fluid passes through a nozzle, where its pressure is dissipated in accelerating the fluid to a high velocity as it exits the mouth of the nozzle. The high velocity fluid stream exiting the nozzle entrains relatively low pressure fluid introduced at a suction inlet to the ejector. Entrainment of the low pressure suction fluid with the motive fluid causes the suction fluid to move with the motive fluid. The two streams mix as they pass into a diffuser portion of the ejector. The velocity profile of the stream changes along the fluid path of the ejector, and the pressure of the stream rises as the fluid reaches the ejector outlet. As the motive fluid flow rate increases, the motive pressure must also be increased to maintain constant discharge pressure due to the increased pressure drop across the ejector nozzle. As the motive/discharge pressure increases, the suction fluid flow rate also increases.